Cells cycle through various stages of growth, starting with the M phase, where mitosis and cytoplasmic division (cytokinesis) occurs. The M phase is followed by the G1 phase, in which the cells resume a high rate of biosynthesis and growth. The S phase begins with DNA synthesis, and ends when the DNA content of the nucleus has doubled. The cell then enters G2 phase, which ends when mitosis starts, signaled by the appearance of condensed chromosomes. Terminally differentiated cells are arrested in the G1 phase, and no longer undergo cell division.
The hallmark of a malignant cell is uncontrolled proliferation. This phenotype is acquired through the accumulation of gene mutations, the majority of which promote passage through the cell cycle. Cancer cells ignore growth regulatory signals and remain committed to cell division. Classic oncogenes, such as ras, lead to inappropriate transition from G1 to S phase of the cell cycle, mimicking proliferative extracellular signals. Cell cycle checkpoint controls ensure faithful replication and segregation of the genome. The loss of cell cycle checkpoint control results in genomic instability, greatly accelerating the accumulation of mutations which drive malignant transformation. Thus, modulating cell cycle checkpoint pathways and other such pathways with therapeutic agents could exploit the differences between normal and tumor cells, both improving the selectivity of radio- and chemotherapy, and leading to novel cancer treatments. As another example, it would be useful to control entry into apoptosis.
It is also sometimes desirable to enhance proliferation of cells in a controlled manner. For example, proliferation of cells is useful in wound healing and where growth of tissue is desirable. Thus, identifying modulators which promote, enhance or deter the inhibition of proliferation is desirable.
Continuous cell proliferation, as in cancer, requires the replication of DNA including chromosome ends known as telomeres. Telomeres decrease in size with successive cell divisions. Also, the number of divisions a cell is capable of negatively correlates with telomere length, and a cell cannot divide once a critical telomere length has been reached. Further, the normal process of telomere shortening with successive cell divisions appears to be circumvented in cancer, suggesting the maintenance of telomore length may be critical to normal and oncogenic growth.
The synthesis of telomeres involves unique DNA replication mechanisms. These mechanisms act to extend telomeres prior to cell division, and are critical to the determination of telomere length in daughter cells. Several molecules involved in telomere synthesis have been identified, including the proteins telomerase, TRF-1 and tankyrase. These and other molecules involved in telomere synthesis provide unique targets for intervention strategies designed to modulate cell proliferation.
Recognized herein is that two aspects of cell proliferation control, namely check point modulation and telomere maintenance, are coordinately regulated and may intersect in some aspect. The present application sets forth tankyrase h nucleic acids and proteins which, without being bound by theory, appear to bridge the gap that currently exists between these two points of control.
Despite the desirability of identifying cell cycle components and modulators, there is a deficit in the field of such compounds. Accordingly, it would be advantageous to provide compositions and methods useful in screening for modulators of the cell cycle. It would also be advantageous to provide novel compositions which are involved in the cell cycle.